The present invention relates generally to a method for reprocessing and separating spent nuclear fuels contaminated with fission products, in a molten metal solvent. More particularly, the present invention is directed to a method for processing and separating spent nuclear fuels wherein a first catalyst is added to the molten metal solvent to promote the carbothermic reduction of actinide fuels which are present as oxides; and a second catalyst is included to accelerate the nitriding of actinides in the solvent.
A molten metal solvent, usually tin, has been used as the solvent and reaction medium for the reprocessing and separation of spent nuclear fuel elements, including actinide fuels, e.g., for the removal of fission products and other impurities from spent uranium-plutonium and thorium-uranium (plutonium) fuels in the metal, oxide, or carbide form. Initially, the spent fuel is declad, if necessary, then put into a solution of molten tin maintained at a temperature of about 1600.degree. C. If the fuel is an oxide, a carbothermic reduction process is necessary. For this purpose, the separation vessel housing the molten tin is comprised of a material which is a source of carbon, preferably graphite. Carbon dissolves in the molten tin and reacts with the actinide and fission product oxides, converting them to a metallic solution and generating CO gas. For a uraniumplutonium fuel the principal reaction is represented as: EQU (U,Pu)O.sub.2(s) +2 C.sub.(in Sn Solution).fwdarw. (U,Pu).sub.(in Sn solution) +2 CO.sub.(g) ( 1)
During dissolution of the fuel, volatile fission products are released and swept out of the separation vessel by the CO, while the more refractory fission products remain behind in the molten tin solvent. Although the volatile fission products have varying volatilities in the molten tin, they are all removed in one step. Along with the volatile fission products a significant portion of the molten tin solvent also evaporates and is removed from the vessel.
Thereafter, the actinides (in solution) are separated from the majority of the non-volatile fission products remaining in the molten tin through a nitriding reaction. A non-oxidizing nitrogen containing atmosphere is introduced into the vessel, resulting in the formation of actinide nitrides in the molten tin. For a uranium-plutonium fuel the reaction is represented by the following equation: EQU (U,Pu).sub.(in Sn solution) +1/2N.sub.2(g) .fwdarw.(U,Pu)N.sub.(s) ( 2)
During the nitriding process molten tin may also evaporate and escape from the separation vessel. Once nitriding is complete, the solid actinide nitrides are separated from the molten tin and the fission products remaining in the tin solvent.
U.S. Pat. Nos. 3,843,765 dated Oct. 22, 1974 to Anderson et al., and 3,843,766 dated Oct. 22, 1974 to Anderson et al. are illustrative of such molten tin separation methods and apparatus.
The solubility of carbon in molten tin is exceedingly slight. For this reason, carbothermic reduction of actinide fuels which are oxides take at least 24 hours. By increasing the amount of available carbon in solution, the amount of time necessary for complete carbothermic reduction may be substantially reduced. Additionally, by increasing the affinity for nitrogen in the metal solvent, the rate at which actinides form solidified nitrides is increased.